1. Field of the Invention
The present invention relates to a method of forming an oxide film and further a method of manufacturing an electronic device utilizing an oxide film.
2. Description of the Prior Art
In recent years, researchers are attracted by thin film transistors utilizing non-single crystalline semiconductor thin films.
Conventionally, such a non-single crystalline semiconductor thin film is formed on an insulating substrate by chemical vapor deposition, so that a temperature during the film formation is as low as 450xc2x0 C. or less. Therefore, soda-lime glass, boro-silicate glass, and the like can be used as the substrate.
The thin film transistor recently attracting researchers is a field effect transistor (simply referred to as FET) having the same function as that of MOS FET. The size of the thin film transistor is limited only by the size of the apparatus to be used for formation of a semiconductor thin film constituting the transistor, so that it is easy to form transistors on large-sized substrates. Such large-sized thin film transistors are promising. For example, the large-sized thin film transistors can be used as switching elements of liquid crystal displays having a lot of pixels in the form of matrix or switching elements of one dimensional or two dimensional image sensors or the like.
It is possible to implement a conventional fine processing to the semiconductor thin films. Hence, the thin film transistor can be formed by means of a conventional fine processing, for example photolithography technique. And it is also possible to make the thin film transistor integrated as a function element of a part of monolithic IC.
Referring to FIG. 2, a typical structure of a conventional thin film transistor is schematically illustrated.
Source and drain electrodes 24 and 25 are provided on an insulating substrate 20 made of glass and source and drain regions 22 and 23 are provided on the source and drain electrodes 24 and 25 respectively and a non-single crystalline semiconductor thin film 21 is provided on the substrate 20 and a gate insulating film 26 is provided on the semiconductor thin film 21 and a gate electrode 27 is provided on the gate insulating film 26.
In the thin film transistor, electric current flowing between the source region 22 and the drain region 23 is controlled by a voltage applied to the gate electrode 27.
A gate oxide film constituting such a thin film transistor was conventionally formed by exposing a semiconductor material to thermal oxidation or by thermal CVD under a reduced or atmospheric pressure, or the like.
Electric characteristics of the thin film transistor largely depend on the quality of a channel region of the semiconductor film and the quality of the gate insulating film. For this reason, a gate insulating film of particularly good quality has eagerly been required.
In the case of the formation of the gate oxide film by exposing a semiconductor material to thermal oxidation or by thermal CVD under a reduced or atmospheric pressure, the temperature during the formation of the gate insulating film should be as high as approximately 600xc2x0 C. in order to obtain a thin film transistor of good electric characteristics. So that, a heat-resistant substrate material such as quartz glass had to be utilized though it is expensive.
With respect to a method for forming a gate insulating film at a low temperature, a plasma CVD and a sputtering method utilizing an argon gas for sputtering are well-known. This sputtering method is implemented in an atmosphere comprising a large amount of argon, specifically an atmosphere comprising 100 to 80 volume % Ar atoms and 0 to 10 volume % oxygen. This is because probability of an atom or a cluster of atoms being dislodged from a target by collision of one inert gas atom, for example one Ar atom, is high (in other words, sputtering yield of Ar gas is high). However, in both the plasma CVD and the sputtering method using a large amount of argon, the gate insulating film involves numbers of elements (e.g. inert gas elements such as Ar) which was involved in a target or existed in a chamber during the CVD or the sputtering, resulting in generation of fixed electric charges in the gate insulating film. Further, ions of the elements bombard a surface of an activated layer in a thin film transistor and thereby give a damage thereto. Hereupon, a mixed layer of the activated layer and the gate insulating film is formed in the vicinity of an interface between the activated layer and the gate insulating film. As a result, interfacial level is formed at the interface and a thin film transistor of fine characteristics cannot be obtained by any of those methods.
It has been attempted to form a gate insulating film by a photo CVD method, and an interfacial level density of the gate insulating film was about 2xc3x971010eVxe2x88x921cmxe2x88x922, almost the same as that of a thermal oxidation film. However, the photo CVD method required a long period of time, in other words, the film formation speed was extremely slow, so that the photo CVD method was not suitable for an industrial application.
Referring now to FIG. 7, a network of silicon oxide formed by sputtering in an atmosphere comprising a large amount of argon is illustrated. Symbols O in the drawing indicate oxygen or silicon and symbols X indicate dangling bonds of silicon. A silicon oxide film including a gate insulating film is not dense when quantity of fixed electric charges is large in the silicon oxide film. The larger the number of dangling bonds of silicon in the silicon oxide film is, the larger the quantity of fixed electric charges is. And the larger the number of Ar+ in the silicon oxide film is, the larger the quantity of fixed electric charges is. Ar+ and Ar tend to stay inside the silicon oxide network as illustrated in FIG. 7 (Ar+ and Ar do not tend to be substituted for Si or O in the network). In fact, numbers of dangling bonds of silicon tend to be generated in the silicon oxide film when the silicon oxide film is formed by sputtering in an atmosphere comprising a large amount of argon. This is partly because internal stress is generated in the silicon oxide film by Ar or Ar+ present inside the silicon oxide network and partly because defects are formed in the silicon oxide film by bombardment of argon with the silicon oxide film during sputtering.
It is an object of the present invention to provide a method of forming a dense oxide film by sputtering.
It is another object of the present invention to provide a method of forming a dense gate oxide film by sputtering.
It is another object of the present invention to provide a method of manufacturing a thin film transistor of high performance at a low temperature.
It is another object of the present invention to provide a method of manufacturing a thin film transistor of high reliability at a low temperature.
It is another object of the present invention to provide a method of manufacturing a thin film transistor of high performance at low cost.
It is a further object of the present invention to provide a method of manufacturing a thin film transistor of high reliability at low cost.
An oxide film in accordance with the present invention is formed by sputtering, so that the formation thereof can be carried out at a low temperature.
A gate oxide film in accordance with the present invention is formed by sputtering, so that the formation thereof can also be carried out at a low temperature.
The sputtering is implemented in an atmosphere comprising an inert gas and an oxide gas or an atmosphere comprising an inert gas, an oxide gas, and a gas including halogen elements, wherein the proportion of the inert gas is small in the atmosphere. If the inert gas occupies a large proportion of the atmosphere during sputtering, the formed oxide film involves numbers of inert gas elements, which results in generating fixed electric charges in the oxide film. In particular, in the case of sputtering in an atmosphere comprising much inert gas of large mass such as argon, the inert gas bombards the oxide film during the film formation and causes a lot of defects in the oxide film. As a result, fixed electric charges are generated due to the defects.
When a soda-lime glass, which is cheap, is used as a substrate, a device formed on such a substrate should be manufactured at a low temperature so that the high performance and the reliability of the device are not degraded by the soda-lime glass. In manufacture of a device comprising an oxide film, the oxide film may be formed by sputtering in accordance with the present invention or subsequently may be further annealed by means of laser or laser pulse. Further in manufacture of a device comprising a semiconductor layer, the semiconductor layer may be annealed by means of laser or laser pulse. The oxide film and the semiconductor layer are not elevated to a high temperature during the laser annealing because a laser energy is very concentrated and also the temperature of the substrate does not exceed 300xc2x0 C. during the laser annealing, so that a cheap soda-lime glass can be used as the substrate.
Concerning the gate oxide film formed by sputtering, a relation between the proportion of the argon gas during sputtering and an interfacial level at the interface between the activated layer and the gate oxide film and a relation between the proportion of the argon gas during sputtering and a flat band voltage were studied. From the study, it was found that both the interfacial level and the flat band voltage largely depended upon the proportion of the argon gas. The interfacial level exerts an influence upon the performance of the gate oxide film.
FIG. 3 is a graphical diagram showing the interfacial level versus the proportion of the argon gas. The proportion of the argon gas in this case means a volume proportion (the argon gas)/(an entire gas comprising the argon gas and oxygen (oxidizing gas)) in an atmosphere during the formation of the gate insulating film constituting an insulated gate field effect transistor by means of sputtering. When the volume proportion is 50% or less, the interfacial level density of the formed film is about {fraction (1/10)} of that in the case of the use of 100% argon atmosphere as apparent in FIG. 3. FIG. 4 is a graphical diagram showing the flat band voltage versus the proportion of the argon gas. A silicon oxide film was formed on a silicon semiconductor by the method of the present invention, and then an aluminum electrode of 1 mmxcfx86 was formed on the silicon oxide film by means of electron beam deposition, whereby an insulated gate field effect transistor was completed. The proportion of the argon gas in FIG. 4 means a volume proportion (the argon gas)/(the entire gas comprising argon and oxygen (oxidizing gas)) in an atmosphere during the formation of the silicon oxide film (i.e. gate insulating film) by means of sputtering. The flat band voltage depends on the amount of fixed electric charges existing in the gate insulating film. The flat band voltage tends to be large as quantity of the fixed electric charges is large. Also, the flat band voltage tends to be small as quantity of the fixed electric charges is small. As seen in FIG. 4, the flat band voltage corresponding to 0% argon gas atmosphere (i.e. 100% oxygen atmosphere) is 1.0V, which is the value of the flat band voltage of ideal C-V characteristic (referred to as ideal voltage hereinafter). That is, when the silicon oxide film formation is implemented in an atmosphere comprising 0% argon (i.e. 100% oxygen), a device with ideal C-V characteristic can be manufactured.
As described hereinbefore, it is desirable to form a gate insulating film by means of sputtering in an atmosphere comprising less amount of argon.
When the volume proportion is no more than 20%, flat band voltage is close to the ideal voltage as shown in FIG. 4. As seen from FIGS. 3 and 4, it is preferred that, in the case of the sputtering atmosphere comprising an oxidizing gas and an inert gas, the oxidizing gas should occupy no less than 50%, preferably no less than 80%, typically 100%, of the sputering atmosphere. Also it is preferred that, in the case of the sputtering atmosphere comprising an oxidizing gas, an inert gas, and a gas including halogen elements, the gas including halogen elements and the oxidizing gas should occupy no less than 50%, preferably no less than 80%, typically 100%, of the sputtering atmosphere.
Sample A and sample B each of which comprises a P-type single crystalline silicon substrate of 1 to 2xcexa9xc2x7cm, a silicon oxide film involving halogen elements formed thereon by the method of the present invention, and an aluminum electrode (gate electrode) of 1 mmxcfx86 formed on the silicon oxide film were prepared. The sample A and the sample B were then annealed at 300xc2x0 C. With respect to the sample A, BT (bias-temperature) treatment (A) in which a negative bias voltage was applied to the gate electrode of the sample A at 2xc3x97106V/cm at 150xc2x0 C. for 30 minutes was carried out. With respect to the sample B, BT (bias-temperature) treatment (B) which was same as the BT treatment (A) except for application of a positive bias voltage in stead of the negative bias voltage was carried out. The difference between the flat band voltage VA of the sample A after the BT treatment (A) and the flat band voltage VB of the sample B after the BT treatment (B) was as large as 9V (The difference is referred to as xcex94VFB (=|VA-VB|) hereinafter). The reason why the xcex94VFB was as large as 9V is that positive ions such as alkali ions, for example sodium ions, were involved in the samples during the formation of the samples. However, when even a few halogen elements, for example fluorine, was added during the formation of the samples, the value of xcex94VFB was largely reduced. This is because the positive ions such as alkali ions were electrically neutralized by the added halogen elements as shown by the following formulae.
Na++Fxe2x88x92xe2x86x92NaF
Sixe2x88x92+Fxe2x88x92xe2x86x92Sixe2x80x94F
Besides, dangling bonds of silicon can be neutralized by the added halogen elements such as fluorine. It is known that dangling bonds of silicon can also be neutralized by hydrogen. However, Sixe2x80x94H bonds obtained by the neutralization are again decomposed by a strong electric field (e.g. BT treatment), so that dangling bonds of silicon appear again, resulting in an interfacial level. Therefore, neutralization by the use of fluorine is preferred.
FIG. 5(A) is a graphical diagram showing a relation between xcex94VFB and a proportion of a fluoride gas. Measurement of the xcex94VFB was carried out with respect to samples which had been prepared in the same way as the samples A and B except that formation of silicon oxide film had been carried out by sputtering in an atmosphere comprising a fluoride gas and an oxidizing gas. FIG. 5(B) shows a relation between the proportion of a fluoride gas and dielectric strength which is defined as a voltage gradient in units of V/cm corresponding to a leak current of 1 xcexcA. Measurement of the dielectric strength was carried out with respect to samples which had been prepared in the same way as the samples A and B except that formation of silicon oxide film had been carried out by sputtering in an atmosphere comprising a fluoride gas and an oxidizing gas. In FIGS. 5(A) and (B), the proportion of a fluoride gas means a volume proportion (the fluoride gas)/(the entire gas comprising the fluoride gas and the oxidizing gas) in the atmosphere.
There was dispersion in the relation between the proportion of a fluoride gas and the dielectric strength. In the graphical diagram of FIG. 5(B), dielectric strength values and their dispersion ranges ("sgr" values) are shown. When the proportion of a fluoride gas exceeds 20 volume %, the values of the dielectric strength of the obtained silicon oxide film are lowered and the a values are increased. So that, the proportion of the added halogen elements is preferably no more than 20 volume %, more preferably in the range of 0.2 to 10 volume % in the present invention. According to SIMS (Secondary Ion Mass Spectroscopy), the amount of fluorine in the film was measured to be 1 to 2xc3x971020cmxe2x88x923 in the case of adding fluorine at a proportion (fluorine)/(oxygen) of 1 volume % during the film formation. From this measurement, it is recognized that fluorine is an element easily involved in a silicon oxide film when added during film formation by sputtering. However, as recited hereinbefore, when the fluorine was added too much, e.g. more than 20 volume %, the obtained silicon oxide film was degraded and the dielectric strength of the film was low with large dispersion.
In the present invention, any of RF sputtering method, DC sputtering method, and the like may be adopted as a sputtering method. However, RF magnetron sputtering method is suitable for the purpose of maintaining a stable discharge when a sputtering target is made from oxide of low conductivity such as SiO2 or artificial quartz.
As the oxidizing gas used in the present invention, oxygen, ozone, dinitrogen monoxide (nitrous oxide), and the like are preferable. In the case of ozone or oxygen, oxygen atoms in the ozone or oxygen might be involved in an obtained oxide film, however, the oxygen atoms do not cause fixed electric charges in the obtained film since they are main ingredients of the oxide film. Accordingly an extremely fine oxide film involving less impurity atoms can be obtained. Besides, since the mass of the oxygen atom is less than that of an Ar atom, even if such oxygen atoms collide with an oxide film formed on a substrate during the film formation, there are few defects caused in the oxide film. Therefore, an excellent oxide film can be obtained.
With respect to the gas including halogen elements, a fluoride gas selected from the group consisting of nitrogen fluoride (NF3, N2F4), hydrogen fluoride (HF), fluorine (F2), or fleon gas can be used. NF3 is preferable since it is easily decomposed and it is convenient for use. Alternatively, a chloride gas selected from the group consisting of carbon tetrachloride (CCl4), chlorine (Cl2), hydrogen chloride (HCl), or the like can be used. The proportion of the gas including halogen elements, e.g. nitrogen fluoride, to an oxidizing gas is preferably 0.2 to 20 volume % in the present invention. These halogen elements effectively neutralize alkali ions such as sodium existing in a silicon oxide film and neutralize dangling bonds of silicon by a heat treatment. On the contrary, if the halogen element added to the silicon oxide film is too much, there is a possibility that the silicon oxide film is somewhat removed in the form of a silicon-containing gas, for example SiF4. For this reason, the proportion (halogen element)/(silicon) in the silicon oxide film is preferably 0.1 to 5 atom %.
It is preferred to use materials of high purity during sputtering in order to obtain a gate insulating film containing less impurities. For example, artificial quartz of no less than 4N, such high purity silicon as to be used as a substrate for LSI, and the like are most preferable as a sputtering target.
Besides, it is preferred that a gas of high purity no less than 5N is used for sputtering in order to prevent impurities from entering a gate insulating film.